Friends are generally good for you, and in particular your mental health. At times, it does not, instead it's bad for you, depending on what kind of friends do you have.
It's not the typical bad friends you would think. Parents, teachers, adults, used to tell when you were in your teenage years to always mix with the right crowd, mix with those who stay home and study. How boring right? Indeed. There are only 2 types, those who love to study, and those who lurk around anywhere and then these people tend to do things that is not expected and it becomes every parent's nightmare.
But, this is not typical. The bad one is the one that depend on you too much, not financially, but emotionally. That's the thing about emotional people. When they get too emotional, they are too feely feely. You gotta guess what they are thinking, they depend on you so much that it becomes a burden. Imagine someone that text you all the time, depend on you whenever they wanna go out, and they wanna involve you all the time. Of course, if you have met this friend 20 years ago when you were young, you cannot expect the same 20 years later. Priorities change, directions and paths change, and the person itself would have grown out from that small kid you knew 20 years ago. You no longer see each other everyday, you don't involve them with everything anymore. They have their own friends, they have their priorities, and eventually things will get even less frequent once the settle down. As a good friend, you will need to understand these changes and it's very natural for people to divert their priorities. Unfortunately, this is a fact of life. You like or you don't, you have to accept it. You cannot compare and everyone is different. If you are a sentimental person, the other might not be. You can't blame them.
In a long run, if I were to be caught in this situation, I would think that I will definitely go insane and psychotic. Having life pressure is enough, what more even friends are giving me the burden. This will be very suffocating. Even though, the term best friends, people fall apart. The definition of best friends change when you grow up. Or maybe, does it even exist, I'm not sure.